The spark plug used for ignition of a fuel gas in an internal combustion engine includes a center electrode and a ground electrode that are insulated to each other by an insulator. When a voltage is applied to the center electrode and the ground electrode, a spark discharge occurs in the clearance between the center electrode and the ground electrode, and the energy of that spark discharge causes the ignition to the fuel gas.
As an example, there has been known a spark plug including a cylindrical hollow ground electrode and a member for connecting the ground electrode to a metallic shell (for example, Patent Document 1). In this plug, the center electrode is arranged inside the cylindrical ground electrode, and a spark discharge occurs in the clearance between the outer circumference surface of the center electrode and the inner circumference surface of the ground electrode.